Abstract
The response of a transplanted mammary carcinoma of Wistar rats, to single intraperitoneal doses of cyclophosphamide ( 25–250 mg kg −1 ) has been assayed by measurement of; delay in growth of the tumour between 9 and 25 mm; cell survival in vitro, 3 and 24 hr after in situ treatment; tritiated thymidine autoradiography and Feulgen microdensitometry of histological material. All three methods of assay indicate that at 9 mm diameter the tumour contains a subpopulation of malignant cells relatively resistant to cyclophosphamide doses in excess of 50 mg kg −1 . The in vitro cell survival assay suggests that these cells comprise 40–50% of the clonogenic population and are capable of considerable in situ repair of potentially lethal damage within 24 hr of treatment. Cellular repopulation from this resistant component, after 150 mg kg −1 of drug, is delayed for 6 or more days and restoration to near pre-treatment status only achieved by 10 days, in this rapidly-growing tumour.
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